According to the American Diabetes Association, there are roughly 14 million diabetics in the United States, with about 7 million having been diagnosed. Of the seven million diagnosed patients, about 105 thousand are treated with injectable insulin, and the rest with weight loss, diet and oral antidiabetic drugs.
Some of the injectable insulin dosage forms are a crystalline suspension, an amorphous, non-crystalline suspension and a combination suspension containing both crystalline and amorphous forms. These are provided in a generally bell-shaped vial. Prior to each injection, the vial is shaken to agitate the liquid suspension, thereby putting any crystals or amorphous particles which may have settled, back into suspension. Because of the vial's shape and because there is gas inside the vial along with the liquid suspension, little agitation of the vial is required to insure sufficient re-suspension of the suspension particles.
With the introduction of pen-like syringes, insulin injection became less complicated and less of a spectacle for the user. Unfortunately, mixing of the insulin to re-suspend the suspension particles became more difficult. The pen-like syringes currently available include a generally cylindrical ampule or cartridge containing the insulin in an essentially bubble-free crystalline suspension. The cartridge has a needle-pierceable cap at one end and a piston or plunger sealing the opposite, open end. The pen-like syringe holds the cartridge, and has a mechanism for advancing a piston rod a pre-determined distance against the bottom side of the cartridge plunger. This forces a corresponding pre-determined dosage of the suspension through the needle. Absent a gas bubble (which is either absent initially or has been ejected from the cartridge prior to the first use), the pen-like syringe and its medicament filled cartridge can be used multiple times to deliver precise, pre-determined dosages. Devices of this type are described in U.S. Pat. Nos. 5,017,190 and 5,104,380.
Because of the shape of the cartridge and the absence of a gas bubble within the cartridge, the ability to quickly agitate the suspension in the cartridge is significantly reduced as compared to the conventional, bell-shaped vial. The contents of the cartridge all have substantially the same density, and shaking, rocking or twisting the cartridge does not impart changes in momentum to the different elements therein sufficient to produce the necessary mixing. One device which has addressed this problem is discussed in U.S. Pat. No. 4,850,966, wherein at least one inert mixing element such as a glass bead is provided inside the cartridge along with the suspension. Because the glass bead has a different density than the suspension, shaking the cartridge causes the glass bead to move within the cartridge and thereby effectively agitate the contents. Unfortunately, the bead takes up space otherwise available for the medicament; the bead reduces the distance that the plunger may be advanced toward the needle end of the cartridge; and the bead represents another element to be manufactured and manipulated in the assembly of the cartridge.
What is needed is an alternative device for insuring proper mixing of the suspension within the cartridge of a pen-type syringe.